This invention relates to a method and apparatus for suppressing the dynamic response of fixed structures to wind, wave or seismic excitation, and under some circumstances, to machinery induced vibration as well.
For example, most fixed offshore structures, such as oil production platforms and light stations, have fundamental flexural and torsional natural frequencies of about 1.0 to 0.2 cycles per second (periods of 1.0 to 5.0 seconds). The natural frequencies are dependent upon the mass distribution and stiffness of the structure and not upon the wind and wave forces. These natural frequencies and occasionally higher mode natural frequencies are excited by random wind and wave forces. Accelerometers can be used to measure the platfrom response, and from the records, the natural frequencies can be determined.
The lowest frequencies of vibration of a structure are the flexural and rotational frequencies associated with the bending and twisting of the entire structure relative to the point of bottom attachment. These frequencies are important for structural reasons, because they are low enough to be driven by the higher-frequency components of the wind and wave spectra. Moreover, because they result in relatively large periodic motions of the entire structure, they represent a significant source of cyclic stress on the major supporting members, thereby decreasing their fatigue life.
The bending or rotational vibration of an offshore tower is usually determined by a low-frequency fundamental with occasional second- and even third-order frequencies superimposed. The lowest frequency usually dominates because it has more energy available from the wind and waves than do the higher-order modes and is therefore the only one of importance when making fatigue life estimates and when designing response suppressing devices. Accelerations caused by the fundamental modes are largest at the top of the tower, where it is relatively easy to find protected locations for the measurement transducers. In the case of a tower with a rectangular platform and symmetric mass distribution, the two flexural motions are parallel to the principal vertical planes of the structure and the rotational motion is about a vertical axis through the geometric center of the structure.
After having identified the significant natural frequencies of the offshore structure, it would be desirable to provide a means for maintaining the response of the structure to the natural forces to which it is exposed. Such a means would increase the life of the structure such as by minimizing the metal fatigue normally caused by the structure's dynamic response to the natural forces.